Nanoparticles, i.e. solids produced as powder or dispersed in liquid media, the individual particles of which extend typically less than 1000 nm in at least two dimensions and which are composed of chemical elements, inorganic or organic compounds or composites, have been used for several years for many applications.
For example, nanoparticles are used as catalyst starting substances, as nanoparticles often have a high BET surface area.
Typically, in what is called the wet-chemical synthesis of nanocrystalline powders or nanoparticles starting from so-called precursor compounds, a powder is synthesized by chemical reactions for example by means of precipitations or by the hydrolysis of organometallic compounds. The final structure of the nanoparticles is usually not obtained until after or during a subsequent calcination following the precipitation.
The targeted control of the crystallization process can be influenced by the composition of the educt(s). An important factor here—in particular when used in the catalysis of such nanoparticles—is the crystallite size (R. Schlögel et al., Angewandte Chemie 116, 1628-1637, 2004). However, it has hitherto been almost impossible, by means of wet-chemical production methods, in particular by means of the so-called basic hydroxide precipitation, to accordingly optimize the precipitation method in respect of the desired nanoparticles and their properties, in particular their average particle-size distribution.
Mechanical production methods for the synthesis of nanoparticles have therefore also been considered. This is carried out for example by intensive grinding of inhomogeneous particles to homogeneous particles, which however often also leads to undesired phase transformations to the point where particles become amorphous due to the pressure exerted on the particles.
However, the particles formed in the process are not present in a uniform homogeneous size distribution, because the very small particles formed display a marked tendency to reagglomerate.
Further access to nanoparticles is via thermophysical methods as disclosed for example in WO 2004/005184. These are typically based on the introduction of thermal energy into solid, liquid or gaseous starting compounds. Here, the so-called plasmapyrolytic spraying process (PSP) of the abovenamed WO 2004/005184 in which the starting substances are sprayed in an oxyhydrogen flame and decomposed is particularly widely used. A typical application for the PSP process is in the production of nanocrystalline silicon dioxide in which readily volatile organosilicon compounds are sprayed in an oxyhydrogen flame.
Furthermore, in the synthesis of nanoparticles the so-called plasma synthesis method is often used in which the starting substances are evaporated in a plasma up to 6,000 K in temperature. Further methods known from the state of the art are for example CVD methods in which gaseous educts are reacted, wherein typically non-oxidic powders and mixed oxidic compounds with different phase structures also form.
Nanoparticulate compositions are also widely used for example in the production of electrode materials for secondary batteries as described for example in EP 1 553 647 A1. In this patent application, in particular the production of lithium iron phosphate is described, wherein the material forms mostly aggregates>1 μm in size. Particles that are as finely dispersed as possible are also be desirable in particular for use as cathode material.
Similar problems result in the production, disclosed in US 2002/0192137, of nanoscale and submicron particles in a flow reactor by means of laser irradiation, wherein complex oxides such as for example lithium phosphorus oxide nitride, lithium iron manganese phosphate, calcium phosphate, aluminium phosphate etc. also form. According to US 2002/0192137, these nanoparticles are likewise used as material for battery applications. A production of in particular lithium iron phosphates by the aqueous route in order to obtain nanoparticles is disclosed in WO 2006/116251, but in the case of lithium iron phosphate a precipitation by means of hydroxide is advised against and the so-called carbonate precipitation is recommended for the starting materials.
The production of lithium iron phosphate (LiFePO4) is known in particular from U.S. Pat. No. 5,910,382 by Goodenough et al. This material is currently the most promising material for use in secondary lithium ion batteries. Furthermore, WO 02/27823 and WO 02/27824 describe the production of LiFePO4 starting from iron phosphate by reaction with lithium carbonate in the presence of a carbon monoxide atmosphere.